Some teenagers may think inadequate sleep is not a problem, but a recent study should serve as a wake-up call to change their lifestyle. US researchers have found that adolescents who went to bed at 10pm or earlier were 24 per cent less likely to suffer from depression than those who slept at midnight or later. The study, conducted by the Columbia University Medical Centre, covered more than 15,500 teenagers in New York. It was published in this month's issue of the journal Sleep. The participants were asked about their bedtimes, hours of sleep and possible symptoms of depression, among other questions. A lack of sleep could affect a teenager's brain functions and lead to moodiness that hinders their ability to cope with daily stress, says study leader Dr James Gangwisch, an assistant professor at Columbia University. This could affect their judgment, concentration and emotions, he adds. According to Professor Albert Martin Li, of the Department of Paediatrics at the Chinese University of Hong Kong, recent research has also revealed a link between inadequate sleep and obesity. Li, who is also honorary secretary of the Hong Kong Society of Sleep Medicine, says children who sleep less are more likely to be overweight. He says children with late bedtimes during weekdays who compensate for their lack of sleep during weekends or holidays are less likely to put on weight than those who never make up the shortfall. Adolescents need at least 10 hours of sleep every day, whereas children aged two to five years need as many as 13 to 15 hours a day, he says. 'But most teenagers sleep only six to seven hours a day,' Li says, adding that many of them stay up to do their homework, surf the internet or play computer games. 'That's a bad habit. When we sleep, metabolism is at work and chemicals and substances we need are brought to where they are needed. So when there is a lack of sleep, we don't get the necessary chemicals in our body, like hormones.' It's likely that many bad-tempered youngsters did not get enough sleep the night before, he says. 'A lot of them sleep little on weekdays and sleep a lot more during weekends and holidays. They think it compensates for their lack of sleep, but it doesn't. In terms of sleep, both quality and quantity are equally important.' Ideally, teenagers should have regular bedtimes and enough sleep, Li says. He also encourages people of all ages to establish a less-lively bedtime routine, like listening to music, reading a book or taking a bath. But is too much sleep bad for us? 'No. some people suffer from narcolepsy [an illness which causes a sudden desire to sleep]. In general, there is nothing bad about having adequate sleep,' Li says.